Nonaqueous electrolyte secondary batteries have high energy density, and are thus widely used as power supplies of portable electronic devices such as mobile phones and laptop computers.
Out of nonaqueous electrolyte secondary batteries, lithium ion secondary batteries have a high voltage of 3.6 V, and may be 50% by mass and about 20-50% by volume of nickel-hydrogen batteries when compared at the same power generation energy. The lithium ion secondary batteries have high energy density and can be thus miniaturized. Furthermore, without memory effect, lithium ion secondary batteries account for a largest share of power supplies of mobile phones and laptop computers.
Small lithium ion secondary batteries are divided into those in a cylindrical shape and those in a flat shape. The smaller, the more easily the batteries are fabricated in the cylindrical shape. As shown in Patent Document 1, a cylindrical lithium ion secondary battery accommodates a wound electrode body inside a cylindrical battery can. The battery can is formed by fixing a sealing plate to an opening of a closed-end cylindrical body with an insulating member interposed therebetween by crimping. The wound electrode body is formed by interposing a separator between positive and negative band-like electrodes and winding them in whorls. The wound electrode body has a through-hole in a winding core center.
A positive electrode current collector plate is welded onto the end edge of the positive electrode of the wound electrode body. The top of the lead extending toward the positive electrode current collector plate is welded onto the back surface of the sealing plate. A positive electrode terminal projecting outside is formed in the sealing plate. On the other hand, a negative electrode current collector plate is welded onto the end edge of the negative electrode of the wound electrode body (i.e., the bottom of the battery can). The surface of the negative electrode current collector plate is welded onto the bottom of the closed-end cylindrical body. A plurality of projections extending to the bottom of the closed-end cylindrical body are formed concyclically on the surface of the negative electrode current collector plate in a portion corresponding to the through-hole of the wound electrode body.
In welding the negative electrode current collector plate onto the bottom of the closed-end cylindrical body, a columnar electrode rod is inserted into the through-hole of the wound electrode body so that the top of the electrode rod comes into contact with the back surface of the negative electrode current collector plate, and so that an electrode piece comes into contact with the back surface of the closed-end cylindrical body to face the electrode rod, with the negative electrode current collector plate accommodating the wound electrode body inside the closed-end cylindrical body. By flowing a current between the electrode rod and the electrode piece, the negative electrode current collector plate and the closed-end cylindrical body are resistance-welded. As a result, the negative electrode current collector plate and the closed-end cylindrical body are jointed and electrically coupled together at junctions between the tops of the projections of the negative electrode current collector plate and the bottom of the closed-end cylindrical body.